A Subscriber Identity Module (SIM) is a removable smart card for mobile phones. SIM cards securely store the service-subscriber key (IMSI) used to identify a GSM subscriber. The SIM card allows users to change phones by simply removing the SIM card from one mobile phone and inserting it into another mobile phone.

The use of SIM cards is mandatory in the GSM world. The equivalent of a SIM in UMTS is called the Universal Integrated Circuit Card (UICC), whereas the Removable User Identity Module (RUIM) is more popular in CDMA phones.

SIM cards are available in two standard sizes. The first is the size of a credit card ( 85.60 mm × 53.98 mm x 0.76 mm ). The newer, more popular miniature-version has a width of 25 mm , a height of 15 mm , and a thickness of 0.76 mm .

W-SIM is a SIM card which also integrates core cellular technology into the card itself.

Memory storage size

The typical low cost SIM card (GSM 11.11 only) has little memory, 2-3 KB as described in GSM 11.11 (telephone directory and so on). Such data storage is used by the phone directly. The market segment of low cost SIM is constantly shrinking.

SIMs with additional applications (GSM11.14) are available in many storage sizes, the largest being the 1 GiB SIM. Smaller sized SIMs such as the 32 KB and 16 KB are the most prevalent in areas with less-developed GSM networks. There are also Large Memory SIMs, on the order of 128-1024 megabytes.

SIM card memory will vary across mobile carriers.

At the end of 2006 the most common GSM SIM in the US is 64 KB - this stemmed from Nov 2004 when the Cingular and AT&T merger triggered the supply of 64 KB SIMs over 32 KB to better support both networks (and make them look like one).

Operating systems

SIM operating systems come in two main types: Native and Java Card. Native SIMs are based on proprietary, vendor specific software whereas the Java Card SIMs are based on standards, particularly Java Card which is a subset of the Java programming language specifically targeted for small embedded devices. Java Card allows the SIM to contain programs that are hardware independent and interoperable.

Data

SIM cards store network specific information used to authenticate and identify subscribers on the Network, the most important of these are the ICCID, IMSI, Authentication Key (Ki), Local Area Identity (LAI) and Operator-Specific Emergency Number. The SIM also stores other carrier specific data such as the SMSC ( Short Message Service Center ) number, Service Provider Name (SPN), Service Dialing Numbers (SDN), Advice-Of-Charge parameters and Value Added Service (VAS) applications. (look to GSM 11.11)